Systems built from integrated circuits generally include capacitors, such as decoupling capacitors between a power supply and ground. A decoupling capacitor provides a low impedance path that helps keep the voltage between the power supply and ground at its nominal voltage despite rapidly varying load demands.
Signals within and outside integrated circuits are frequently referenced to a power plane or a ground plane, and a driven signal induces a corresponding return signal in the nearby portions of the reference plane. A via can change the reference plane for a signal. In one example, the top two layers of a printed circuit board are an upper signal layer on an inner power plane and the bottom two layers of the printed circuit board are a lower signal layer on an inner ground plane, and a via transfers the signal from the upper signal layer to the lower signal layer. This via changes the signal's reference plane from the power plane to the ground plane. When a signal has a via that changes the reference plane, the induced return signal must switch reference planes. A decoupling capacitor between two reference planes provides a low impedance path for high-frequency components of the induced return signal to switch between the two reference planes.
Ignoring the effects of parasitic inductance, at all frequencies a large decoupling capacitance has a lower impedance than a small decoupling capacitance. However, a large decoupling capacitance generally requires a physically larger capacitor and longer wires to connect a load circuit to the physically larger decoupling capacitor. These longer wires produce parasitic inductance that limits the usefulness of a large decoupling capacitor.
A decoupling capacitor's capacitance and in-circuit parasitic inductance define a resonant frequency. A decoupling capacitor is often most effective near its resonant frequency, because both the capacitance and the parasitic inductance have a low impedance near the resonant frequency. Above the resonant frequency, the parasitic inductance produces a high impedance that limits the effectiveness of the decoupling capacitor. Below the resonant frequency, the capacitance of the decoupling capacitor produces a high impedance in comparison with a larger decoupling capacitor. Thus, in systems producing power supply noise over a wide frequency range, cost-effective power supply decoupling generally uses a range of sizes of decoupling capacitors.
The wiring between a decoupling capacitor and its load circuit forms a loop of a one-turn inductor, and the cross-sectional area of the loop roughly gives the parasitic inductance of the loop. Keeping the outgoing and incoming sides of the loop close together reduces the parasitic inductance, but the distance between the decoupling capacitor and its load circuit ultimately provides a lower bound on the parasitic inductance. Thus, the decoupling capacitors for bypassing high-frequency noise should be close to the load circuit that generates the high-frequency noise.
It is especially difficult to filter high-frequency noise created by the output circuits of an integrated circuit. These output circuits typically drive loads through wiring on a printed circuit board. Such remote loads have their own current loop producing parasitic inductance that is generally much greater than the parasitic inductance of loops confined within internal circuits of the integrated circuit. Moreover, in many integrated circuits, the input/output circuits drive remote loads at a higher switching rate than the bulk of the internal circuits of the integrated circuit. Thus, the input/output circuits of many integrated circuits have both a higher parasitic inductance and a higher generated noise frequency than the internal circuits of these integrated circuits. This makes effective decoupling capacitors especially difficult for the output circuits of many integrated circuits.
For high-frequency input signals, it is especially difficult to provide a low impedance path to allow the induced return signal to change reference planes between a power plane and a ground plane. A decoupling capacitor provides this function, but may typically be located on the package substrate or on the printed circuit board where the loop area between the input transistors and the decoupling capacitor creates a large, undesired, inductive parasitic element that limits high-frequency signal integrity.